U.S. Pat. No. 4,484,179 issued to L. R. Kasday discloses, inter alia, an optically-based touch-sensitive touch screen for determining the position of a finger touch on the surface of a CRT screen. This arrangement includes a transparent, compliant surface which overlays the surface of the CRT screen and through which light from the CRT may pass. When a point on the compliant overlay is touched, light from the phosphorescent CRT screen becomes trapped within the overlay by total internal reflection. This trapped light travels to the edge of the overlay where it is detected by photodiodes. Since total internal reflection of light originating from a point under the finger occurs at the time the CRT raster beam passes under the finger, the raster beam timing can be used in conjunction with the signals outputted by the photodiodes to determine the position of the touch.
The level of light signals that become trapped within the compliant overlay when it is touched is a function of the degree to which the overlay deflects. Disadvantageously, then, a relatively non-compliant overlay will only trap a low level of light signals, resulting in a relatively low photodiode signal level. This, in turn, would necessitate expensive signal detection circuits to process the low-level photodiode signals, thereby adding to the cost of the optically-based touch screen.
Accordingly, a desirable characteristic of an optically-based touch screen is that it deflects substantially when it is touched by the user with a relatively low level of force, thereby increasing the level of trapped light and, hence, the level of the resulting photodiode signals to a point where the latter signals can be processed inexpensively. The above-mentioned Kasday patent suggests using silicone rubber for the compliant overlay. While this material is somewhat soft, the user nonetheless must apply an uncomfortable level of force to the silicone material to increase the level of trapped light to a point where the resulting photodiode signals can be processed using inexpensive detection circuits. Although materials that are softer than silicone rubber are easy to deflect and hence trap a higher level of light signals when they are touched, they are, for the most part, nonetheless unacceptable as a compliant overlay, since such materials disadvantageously tear easily when they are stressed and are also tacky. A material that is tacky sticks to the surface of the CRT screen and is easily marked by fingerprints.